Many wireless communication protocols provide for transmitters, operating within a communication network, which are capable of employing phase modulation (PM) techniques and (AM) amplitude modulation techniques. Examples of such wireless communication protocols include Enhanced Data Rates for Global Systems for Mobile Communications Evolution (EDGE), Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA). To accommodate the modulation requirement for each particular communication protocol, wireless devices such as wireless telephones, wireless personal data assistants (PDAs), pagers, two-way radios and other types of wireless devices employ a transmitter and modulation circuitry for providing the required amplitude and phase modulation.
FIG. 1 is a block diagram of a transmitter stage 10 illustrating one previous technique, which employs a single loop amplitude modulation circuit 110, a phase modulation control circuit 120 and a base band processor 122. The single loop amplitude modulation circuit 110 includes an amplifier 124, an RF coupler 126 an envelope detector 128, and an amplitude modulation feedback circuit 129. The amplitude modulation feedback circuit 129 includes a capacitor 130 and a difference amplifier 132. A portion of the RF output signal 134 produced by the amplifier 124 is fed to the envelope detector 128 via the RF coupler 126 as an RF coupled output signal 136. The difference amplifier 132 generates a power control signal 142 based on the difference in voltage between the detected envelope signal 144 and an amplitude modulation signal 140 provided by the base band processor 122. The base band processor 122 may include a digital to analog converter to produce the amplitude modulation signal 140 for conversion from digital data to an analog signal. The difference amplifier 132 provides the power control signal 142 to the amplifier 124 such that the amplitude of the RF output signal 134 from the amplifier 124 is responsive to the power control signal 142, thereby achieving amplitude modulation of the RF output signal 134.
The phase modulation control circuit 120 consists of a mixer 146, a phase comparator circuit 148, a voltage controlled oscillator (VCO) 150, a switch 152, and a limiter circuit 153. The switch 152 selects from either a synthesizer output signal 154 produced by the VCO 150 or the RF coupled output signal 136 and provides a feedback signal 155 to the limiter circuit 153. Before the amplifier 124 is turned on, the switch 152 couples the synthesizer output signal 154 from the VCO 150 to the mixer 146 via the limiter circuit 153 as a limited feedback signal 156. After the amplifier 124 is turned on and the single loop amplitude modulation circuit 110 attains a locked condition, the switch 152 couples the detected RF output signal 136 to the mixer 146 via the limiter circuit 153. Accordingly, the switch 152 receives the detected RF output signal 136 and in response produces the feedback signal 155 such that the amplifier 124 becomes part of a phase locked loop formed by the phase modulation control circuit 120 and the single loop amplitude modulation circuit 110. As a result, the phase modulation control circuit 120 compensates for the phase distortion of amplifier 124.
The mixer 146 generates a phase difference signal 158 having an averaged energy level that is equal to the difference between the phase of the frequency reference signal 162 and the limited feedback signal 156. The phase comparator circuit 148 generates a modulated phase difference signal 160 based on the phase difference signal 158 and a phase modulation signal 161. The phase modulation signal 161 is provided by the base band processor 122. VCO 150 receives the modulated phase difference signal 160 and in response produces the synthesizer output signal 154. Since the limiter circuit 153 and the switch 152 have low phase distortion, the RF output signal 134 has a phase that is approximately equal to the phase of the frequency reference signal 162. When switch 152 is switched to receive the RF coupled output signal 136, then the phase modulation control circuit 120 thereby achieves phase modulation of the RF output signal 134.
A problem arises when the power control signal 142 provided to the amplifier 124 for controlling the amplitude of RF output signal 134 causes a phase shift on the RF output signal 134, referred to herein as an AM to PM conversion effect. A significant phase shift may occur, requiring burst-to-burst calibration due to the AM to PM conversion effect. This AM to PM conversion effect is a result of a nonlinearity of the amplifier 124 that is characteristic of power amplifiers that employ design techniques used to minimize bias current and maximize power efficiency and when the power of the RF output signal 134 is controlled by varying the amplifier 124 bias current. However, such techniques to enhance operating efficiencies may cause phase distortion in the RF output signal 134, resulting in significant errors in a receiver when attempting to receive the phase distorted RF output signal 134.
In the past, several approaches have been used in an attempt to eliminate this AM to PM conversion effect. According to one approach, by using a more linear amplifier 124, AM to PM effects were reduced. However, a highly linear amplifier 124 is inefficient and power-consuming and is not desirable for applications such as portable wireless devices.
According to another approach, the phase of the frequency reference signal 162 is adjusted such that the synthesizer output signal 154 is phase-predistorted, thereby canceling the phase distortion that occurs in the amplifier 124. However, the required degree of phase predistortion is dependent on the RF output signal 134 level, a supply voltage to the amplifier 124, and an operating temperature resulting in a very complex open loop control scheme. Additionally, the phase predistortion is further complicated when amplitude modulation is employed.
According to another method, the single loop amplitude modulation circuit 110 compensates for amplitude modulation distortion in the amplifier 124. The single loop amplitude modulation circuit 110, however, has a loop bandwidth that varies as a function of the power control signal 142. As a result, the single loop amplitude modulation circuit 110 may not sufficiently compensate for the amplitude modulation distortion within amplifier 124 resulting in an RF output signal 134 having excessive amplitude modulation distortion especially at high data rates.